Digital Video Broadcasting-Terrestrial (DVB-T) standard adopted for terrestrial digital broadcasting in Europe uses an orthogonal frequency division multiplexing (OFDM) technique.
OFDM is a type of multi carrier modulation, and has superior performance in both multi-path and portable receiving environments. Therefore, the OFDM has been noted as a modulation method appropriate for terrestrial digital television and digital audio broadcasting.
OFDM has mainly been researched in the field of communication, but research has been conducted in the field of broadcasting since the OFDM has been adopted by the European Broadcasting Union (EBU) as a digital audio broadcasting system modulation method.
An OFDM transmission signal is generated by combining a plurality of digital modulation waves. The respective carrier wave modulation uses quadrature phase-shift keying (QPSK) for audio broadcasting, and a multi-level modulation such as 64 Quadrature amplitude modulation (QAM) for terrestrial digital TV broadcasting.
When OFDM is used for transmitting data, the basic unit is a symbol. Each symbol includes a valid symbol interval and a guard interval (GI). The valid symbol interval is used to transmit data, and the guard interval is required to reduce interference between multi-path channels.
If the guard interval is long enough, the interference between the multi-path channels is reduced. As data are not transmitted within the guard interval, if the proportion of the guard interval in each symbol is larger than that of the valid symbol interval, the loss of data rate increases.
The length of the guard interval should be maintained to conform to the channel environment in order to alleviate the deterioration in the data transmission rate caused by the use of the guard interval.